I Can’t Drive 55 – A Compact Dilemma

6 06 2010

Sammy Hagar sang about it in his song from 1984. Here, myself and other teammates who run a compact crankset (110 BCD), likewise find ourselves running out of gears when we’re in a pack on a flat or flying downhill. (Mine is a Shimano FC-6650.) What to do?

Having recently replaced my worn-out Shimano 105 11-25t for an Ultegra 12-25t (reason-super low-price and in this case, not a good reason), I can tell the difference in that one tooth in the back. In one Crit and the last road race I’m easily turning over 105 RPM in my highest gear–50/12. Other teammates also remark the same.

So what now? my chainrings have at least 4000 miles on them and they’re worn enough to have the chain ride-up the teeth under tension. I want to keep my low-end gear ratios, but have a bit more high-end than I have now. Here’s how I see it:

Option Pro/Con
(1) Keep the 12-26t, and buy a new 50/34t chainring set. I keep my low-end gear ratios, but still have the high-end issue. Address the high-end issue with increased wattage output and endurance training (free).  Lowest cost option.
(2) Return to a 11-25t, and buy a new 50/34t chainrings. A slightly higher top-end than option (1). The most costly option. Address the high-end issue with increased wattage output and endurance training (free).
(3) Keep the 12-25t, and buy a new 52/34t chainring set. I’ve found only 4 suppliers that have a 52t/110 BCD chainring. Also, shift timing may be an issue between the double chainrings. Cost for the 52t chainring can be high. Additionally, the front derailleur (FD-6600) may not handle an 18t difference. On the other hand, I keep my low-end gears and benefit from a higher top-end. The second highest cost option.
Part suppliers (check with your LBS)
  • Shimano
  • FSA
  • Origin8
  • TA-Specialties
  • Tiso (51t, 110mm, 5 bolt, 7075-T6)
The front derailleur

I run an FD-6600 front derailleur. Shimano’s specification says that the maximum tooth difference for the chainrings is 15. This appears to conflict with reality because 50t less 34t equals a 16 tooth difference, and the front FD shifts just fine. I’ve read comments elsewhere that Shimano’s FD capacity figure is conservative (this comment was just one of a few).

Another issue might be the curvature of the bottom of the outer cage versus the bigger diameter of the 52t chainring. Best shifting performance occurs with the FD mounted as close as possible without the cage hitting the chainring. The Shimano document above says 1 to 3mm is appropriate. If the gap can be thus, then the curvature might not be an issue. I measured the gap at about 1.5mm.

The race courses

Three of the local-area courses have bigger climbs within their routes. (Corsa Brutale, Chapman Lake-long and Double-Trouble.) One of the courses, the Mt. Spokane hill climb has an ascent of 3,975 feet. Most of the courses having rolling hills instead.

Conclusion

For the short-term I’ll go for option (1). Sometime thereafter I’ll replace the cassette as in option (2). Or maybe sell-off the Ultegra cassette soon (relatively low mileage), and use the bucks toward an 11-25t cassette. Either way, I’ve some hard training to do.

Please comment on your experience with this issue.

Post-Race note:

I switched back to my old 105 11-25t cassette for yesterdays Chapman Lake Long course; and was able to use the 11t cog more effectively than the 12t. I expect that my next cassette will be a Shimano CS-6700 in the 11-25t ratio.

Update:

I’m still running the 11-25t cassette. Additionally, I finally found a U.S. importer for the Tiso chainrings. Contact them directly here. Sometime this week I should receive the 51t big-ring. Club rides and races will follow wherein I’ll be able to test my hypothesis. Given positive results, I’ll be able to present my findings to my teammates for their consideration and tactical advantage.

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One response

7 06 2010
cris lucas

54-42 front and a 24-12
1987 is here again
if i’m spinnin out we’re goin’ 40mph
see you tomorrow

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